Method and apparatus for reducing repeatable runout in storage systems

ABSTRACT

A method and apparatus is described for reducing RRO in storage systems. A disc may be partitioned into a number of equally spaced sectors. An RRO profile may be individually obtained for each sector, a runout control algorithm may be applied to each sector to generate an RROC waveform for the sector to suppress the RRO, and sector RROC waveforms may be assembled into an RROC waveform for a whole revolution and saved in a memory buffer for feed-forward control. The RROC is performed in the time domain, and it may be adapted for each sector to reject the RRO disturbance.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to previously filed U.S.provisional patent application Ser. No. 61/054,697, filed May 20, 2008,entitled APPARATUS TO ROBUSTLY REDUCE THE REPEATABLE RUNOUT DISTURBANCESIN DISC BASED SERVO SYSTEM WITH FAULT-TOLERANCE CAPABILITY. Thatprovisional application is hereby incorporated by reference in itsentirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to disc based servo systems, andmore particularly to reducing repeatable runout (RRO) in disc basedservo systems.

2. Description of Related Art

A storage servo system is used to reduce mechanical displacementmismatches between an actual position of a read/write head and a targetposition of the head. The mechanical displacement mismatches, ordisturbances, in a servo system may include RRO and non-repeatablerunout. Repeatable runout may be caused by disc irregularity, disceccentricity and/or spindle axis assembly (mechanical misalignments) andare repeatable in each spindle rotation. Non-repeatable runout is notsynchronized with disc sectors, e.g., unmodeled dynamics.

Some hard disk drives (HDDs) use a frequency domain multiple sine wavesynthesizing method for feed-forward RRO disturbance control. Thismethod needs significant processing power to generate multiple sinewaves and calculate the phase and magnitude for each sine wave tocompensate for repeatable runout.

A removable disc is not fixed to any storage system and may be loadedinto a storage system when needed. Examples of removable disc storagesystems may include CD-ROM drives and DVD drives. Some people believethat disturbances in removable disc storage systems are not predictable,and that repeatable runout control (RROC) in removable storage systemsis impossible, because the recording media is removable and the loadermechanism is low cost. Others are trying to implement RROC in removabledisc storage systems using the above-described magnetic storageapproach, using a high bandwidth feedback control loop with a highsampling rate. However, with the high sampling rate, such an approach isprohibitively expensive.

Therefore, it may be desirable to provide a method and apparatus forreducing RRO in removable disc storage systems.

SUMMARY

A method for reducing repeatable runout (RRO) may comprise: learning anRRO profile of at least two sectors in a removable disc; forming arepeatable runout control (RROC) profile for the at least two sectors;assembling the RROC profile of the at least two sectors into an RROCprofile for the removable disc; and storing the RROC profile for theremovable disc in a first memory to provide a feed-forward controleffort in time domain to suppress the RRO. A removable disc may be,e.g., an optical, a magnetic or a magneto-optical disc, though it is nota limitation of the invention.

In one aspect of the present invention, one of the at least two sectorsmay be a target disc sector, and the method may comprise: providing afeed-forward control effort to suppress the RRO for the target discsector when a head reaches the target disc sector.

The method may comprise: providing a feed-back control effort tosuppress non-repeatable runout for the target disc sector after the headreaches the target disc sector.

The method may comprise: separating RRO from non-repeatable runout forthe target disc sector.

The method may comprise: adapting the feed-forward control effort of thetarget disc sector to match the RRO of the target disc sector.

The method may comprise: storing the feed-forward control effort whichmatches the RRO of the target disc sector in a second memory toadaptively adjust the RROC profile.

The method may comprise: promoting the RROC in the second memory to thefirst memory after a spindle revolution.

The method may comprise: removing a direct current (DC) element of theRROC profile from the RROC profile before the promoting.

The method may comprise: stopping the learning when the head isoff-track.

A servo system may comprise: a first memory for storing a repeatablerunout control (RROC) profile which provides a feed-forward controleffort to suppress repeatable runout (RRO) of a target disc sector of aremovable disc when a head reaches the target disc sector; and acompensator for receiving an error signal and providing a feed-backcontrol effort to reduce non-repeatable runout.

The server system may further comprise: a low pass filter for separatingRRO from the error signal and passing the RRO to the first memory.

The server system may further comprise: a feed-back loop connecting anoutput of the first memory to an input of the first memory to adapt theRROC of the target disc sector.

The server system may further comprise: a second memory for temporarilystoring an RROC profile before a whole revolution is completed.

In one aspect of the present invention, the compensator may comprise aninfinite impulse response (IIR) filter.

The server system may further comprise a phase lock loop (PLL) forpartitioning the disc into a number of sectors.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the present invention are described herein with referenceto the accompanying drawings, similar reference numbers being used toindicate functionally similar elements.

FIGS. 1A and 1B illustrate a method for partitioning a disc intoequally-distributed sectors.

FIG. 2 illustrates a servo control architecture for a removable discstorage system according to one embodiment of the present invention.

FIG. 3 illustrates a servo control architecture for a removable discstorage system with adaptation according to one embodiment of thepresent invention.

FIG. 4 illustrates an exemplary frequency response of a filter used inthe servo control architecture of FIG. 3.

FIG. 5 illustrates an architecture for dual buffer based iterativelearning according to one embodiment of the present invention.

FIG. 6 illustrates a flow chart of a method for reducing RRO accordingto one embodiment of the present invention.

FIG. 7 illustrates a removable disc storage servo system according toone embodiment of the present invention.

DETAILED DESCRIPTION

A method and apparatus is described for reducing RRO in removable discstorage systems. A disc may be partitioned into a number of equallyspaced sectors. An RRO profile may be individually obtained for eachsector, a runout control algorithm may be applied to each sector togenerate an RROC waveform for the sector to suppress the RRO, and sectorRROC waveforms may be assembled into an RROC waveform for a wholerevolution and saved in a memory buffer for feed-forward control. TheRROC is performed in the time domain, and it may be adapted for eachsector to reject the RRO disturbance. The method of the presentinvention may achieve better RRO rejection through an adaptivefeed-forward and feedback servo architecture with minimal controlbandwidth and minimal sampling rate; may reduce implementation costthrough reduced memory usage by employing time domain adaptation forfeed-forward control which may require in one embodiment an array ofmemory buffers; may avoid synthesizing multiple sine waves; and mayimprove system robustness against servo conditions by disabling adaptivelearning during its seek mode, settle mode, defect mode and/or anoff-track servo failure. The invention may be carried out bycomputer-executable instructions. Advantages of the present inventionwill become apparent from the following detailed description.

FIGS. 1A and 1B illustrate a method for partitioning a disc intosectors.

Since repeatable runout at a location on a removable disc, e.g., anoptical disc, may occur each spindle rotation, the disc may be dividedinto a group of sectors (e.g., which may be equally spaced and/orequally sized, but not limited as such) for RROC waveform shaping, asshown in FIG. 1A. Each sector may need a memory to store its RROC value(or profile). Therefore, the greater the number of sectors, the biggerthe memory size required, and the finer the resolution of the RROC. Thenumber of sectors may be a trade-off between the RROC resolution andmemory cost. The embodiment shown in FIG. 1A divides a disc into 128sectors, but the number of sectors may be programmable and may haveother values, e.g., 64 or 32.

Sector partitioning of a disc may be achieved by various methods. In oneembodiment, an angle index phase lock loop (PLL) algorithm may be usedto generate a sequence of signals from a given hardware signal from aloader assembly, so as to partition a disc into equally spaced sectors.A spindle motor driver in a storage loader assembly may provide asequence of pulses, called Frequency Generator (FG), and there may be 18FG pulses per disc revolution for most of the popular motor drive ICs.The rising edges of some FG signals are shown in FIG. 1A. Although theFG pulses may not be evenly distributed in one revolution, the phaseposition of each FG may remain substantially the same for eachindividual loader. The fixed phase position of FG pulses may be used topartition the disc into equally spaced and/or sized sectors.

If the angle (or phase) of a disc is a floating point number (ornormalized value) 1.0, which may be converted into a fixed pointrepresentation by m-bit (for example m=32), the kth sector phaseposition may be k/N, where N is the total number of sectors in onerevolution. The time interval for a sector may be measured with areference clock. In one embodiment, the PLL algorithm may be implementedin hardware, and a faster reference clock, e.g., in the MHz level, maybe chosen to generate a fine resolution of sector partition with fineresolution of jitter on each sector boundary. In one embodiment, the PLLalgorithm may be implemented in software, and a slower reference clock,e.g., 88 KHz, may be used. The normalized phase for the time intervalmay be obtained by dividing the time interval by the time for an entiredisc rotation, regardless of the spindle speed.

As shown in FIG. 1B, an FG measure module 101 may receive a referenceclock and the FG pulses to generate an actual FG phase.

At a comparator 102, the actual FG phase may be compared with an FGphase target to generate a phase error measurement for the PLL. The FGphase target may be the normalized phase position of the FG pulses inthe initial rotation.

A Proportional-Integral (PI) compensator 103 running at a fixed samplingrate (1 KHz) may be used in the PLL to provide a normalized phase error.The transfer function of the PI compensator 103 with a sampling rate of1 KHz may be:

${{PI}( z^{- 1} )} = {\frac{2^{- 18}}{1 - z^{- 1}} + 2^{- 4}}$

A normalized phase accumulator 104 may receive a normalized phase errormeasured at each FG pulse from the PI compensator 103 and a referenceclock, and predict a normalized phase increment adjustment at eachreference clock.

The phase increment at each reference clock may be preset based on thenormalized phase calculation. This phase increment value may be adjustedby the PLL. The PLL may synchronize, or phase lock, the normalized phaseaccumulation in the reference clock domain with the FG phase rotation inthe FG pulse domain.

The spindle speed change may be a main distortion to the normalizedphase definition. Since the PLL bandwidth may be much larger than thespindle speed change, it may be more than enough to use PLL tocompensate for the phase distortion from the spindle speed change. Inother words, the PLL may track the spindle speed change with areasonably small tracking error. As will be described below, theadaptive RROC may tolerate this small PLL tracking error during the discsector partition.

An angle index generator 105 may generate sector angle index pulses fromthe output of the normalized phase accumulator 104 by taking out itsn-MSB.

FIG. 2 illustrates the architecture of a servo control system for aremovable disc storage system according to one embodiment of the presentinvention. The removable disc may be an optical disc, although the servocontrol system may be used with other types of removable discs. Thesystem may be a focus or radial servo system, and may use feedback toreject non-repeatable runout (Dnrro) and feed-forward to rejectrepeatable runout (Drro). A plant of an optical pick-up unit (OPU)system 200 may be modeled by a plant dynamic P 201 and two disturbances:Drro and Dnrro.

A table 202 may be used as a feed-forward controller and may be an RROCprofile. The table 202 may be a static runout profile obtained inadvance in an open loop. As shown in FIG. 1A, a disc may be partitionedinto 128 equally spaced sectors. An RRO profile may be individuallyobtained for each sector, a runout control algorithm may be applied toeach sector to generate an RROC waveform for the sector to suppress itsRRO, and sector RROC waveforms may be assembled into an RROC waveformfor an entire revolution and saved in a memory buffer for feed-forwardcontrol. In one embodiment, the runout control algorithm may generate anopposite waveform for RRO, i.e., by constructing a waveform that is thesame as the RRO profile but with a 180° phase difference, so as toeliminate repeatable or predictable disturbances. The RROC waveform fora sector, which may provide a feed-forward control effort to suppressRRO in the sector, may be saved as an entry in the table 202.Embodiments for generating and adapting the RROC waveform will bedescribed below in more detail with reference to FIGS. 3, 4 and 5.

In operation, the table 202 may receive information indicating thetarget disc sector and provide a control effort n to suppress the Drrofor the target disc sector for feed-forward control when the headreaches the target disc sector. At the same time, a closed loop feedbackcompensator C 203 may receive an error signal e from the plant, whichideally may only include Dnrro if Drro is already successfullysuppressed by the control effort n, and generate a control effort m tocorrect Dnrro. In one embodiment, the compensator C may be a high orderIIR (Infinite Impulse Response) filter.

FIG. 3 illustrates the architecture of a servo control system for aremovable disc storage system with adaptation according to oneembodiment of the present invention. The system shown in FIG. 3 mayadapt the RROC waveform in FIG. 2 to increase the system's robustnessagainst errors, e.g., errors in sector partition and various systemvariations.

In particular, the table 202 in FIG. 2 may be replaced by the memorybuffer, the element of which can be represented by a single variablez^−1 301 with an adaptation law. The adaptation law may be a runoutadaptation feedback loop with a filter Q 303, which runs much moreslowly than the feedback loop with the compensator C 203. The singlevariable z^−1 may correspond to an entry in the table pointed by thetarget disc sector. The variable z^−1 may be updated once per spindlerevolution and the update may be pure accumulation. Similarly to thesystem shown in FIG. 2, z^−1 may provide a feed-forward control effort nto suppress Drro for the target disc sector when the head reaches thetarget disc sector. When the head reaches the target disc sector, thecompensator C may receive an error signal e, which may include Dnrro,and provide a control effort m to suppress the Dnrro.

If the system does not have a runout adaptation feedback loop or therunout adaptation feedback loop can not achieve enough performance, Drromay show up in the error signal e. For example, when the feedback loopwith the compensator C is just closed, the error signal e may includenot only Dnrro, but also Drro. If the error signal e is contaminatedwith Drro, neither the control effort m for Dnrro nor the control effortn for Drro is reliable. Adaptation may be used to learn the true Drroand keep reducing the effect of Drro until the error signal e is free ofDrro.

Since Dnrro is usually high frequency, and Drro is usually lowfrequency, a low pass filter L 302 may be used to pass only Drro toz^−1. In one embodiment, the filter L may be a fourth order IIR filterwith cut-off bandwidth of 2 Khz. Its exemplary frequency response isshown in FIG. 4, and its transfer function may be:

$\frac{0.003036}{1 - {3.24\mspace{14mu} z^{\bigwedge}} - 1 + {3.974\mspace{14mu} z^{\bigwedge}} - 2 - {2.187\mspace{14mu} z^{\bigwedge}} - 3 + {0.4557\mspace{14mu} z^{\bigwedge}} - 4}$

The filter L may also help to reduce aliasing caused by the gap betweena high sampling rate in the feedback loop with the compensator C and alow update rate of the runout adaptation feedback loop. The feedbackloop with the compensator C may be running at a high sampling rate,e.g., 88 KHz, 176 KHz or 352 KHz, and may be independent of the spindlespeed. However, the update rate of the runout adaptation feedback loopwith the filter Q may be much lower than the sampling rate, e.g., about20 KHz for a 16×DVD. In addition, the update rate of the runoutadaptation feedback loop may be determined by a spindle angular speed,since the RRO profile is learned and stored sector by sector, and thesector period may become shorter if the spindle rotates faster.

The filter Q may be an FIR (Finite Impulse Response) filter and may beused in the runout adaptation feedback loop to match the control effortn to Drro. In one embodiment, Q=1, the value of z^−1 may be added to theoutput/from the filter L as a feed-back, and the control effort n maykeep increasing until it can suppress Drro. When the control effort ncan suppress Drro, the error signal e does not have Drro anymore, l=0,z^−1 and n may reach their ideal values and stay there until Drroappears in the error signal e again.

Thus, even though there may be some errors in the control effort n, theadaptation may still adapt to learn and compensate for Drro. As aresult, the runout adaptation feedback loop may provide an accuratecontrol effort n to suppress Drro, and the feedback compensator C mayprovide an accurate control effort m to eliminate the non-repeatablerunout Dnrro, the system may achieve the maximum disturbance reduction,and its robustness against disturbances may be increased.

FIG. 5 illustrates an architecture for dual buffer based iterative RROClearning according to one embodiment of the present invention.

The adaptive RROC learning process described with reference to FIG. 3may be disrupted by various events during drive operation. In oneembodiment, to prevent bad RROC learning results from being stored andused in feed-forward control, the RROC learning may be allowed when theservo is in the track-following mode with good radial lock performance,and stopped when a seek operation or off-track condition happens. In oneembodiment, when an unexpected off-track condition happens, the bad RROClearning results may be replaced by a RROC profile obtained during aprevious rotation.

As shown, the architecture for dual buffer based iterative RROC learningmay maintain two memory buffers for the RROC profile: a learning buffer501 which may be a temporary buffer used to hold real time RROC learningresults; and a working buffer 502 which may be used to hold a matureRROC profile for actual servo control.

After a seek is finished (e.g., indicated by a timing lock or PSNdecode), the track-following may start and the learning process maystart at the same time. The learning process needs to use the workingbuffer 502 obtained during the previous rotation as the base for theRROC learning. Every FIQ (Fast Interrupt Request), the RROC profile maybe output with the output pointer pointing to the target RROC elementdesignated by an angle index K. The working buffer 502 may provide afeed-forward control effort n for the target sector K, while learningthe RRO profile of the sector K during the current rotation.

In one embodiment, when learning the RROC profile for the target sector,its neighboring sectors may be considered to tolerate adaptationmistakes, for example, mistakes in the spindle spin up or spin downprocess. In one embodiment, the value of z^−1 for the previous sectorK−1, the target sector K and the next sector K+1 may be sent to a Qfilter 503, and the Q filter 503 may use a weighted average of thecontrol effort for the three sectors, with the weight for the targetsector having the biggest value. In one embodiment, the weights for theprevious, target and next sector may be 0.25, 0.5 and 0.25 respectively,and the Q filter for the kth sector may be calculated as follows:

${Q_{k} = {\frac{x_{k + 1} + x_{k - 1}}{4} + \frac{x_{k}}{2}}},$

wherein X_(k) is the kth entry in the RROC profile.

The output of the filter Q 503 may be combined with the signal/from thefilter L 302 at 504 and the combination may become an update 505 of z^−1of the target sector. The update of z^−1 may be stored in the learningbuffer 501, and the working buffer 502 may continue to the next discsector K+1. The learning buffer 501 may hold the temporary RROC profileduring the real time learning, but the temporary RROC profile may not beused for servo control.

When the learning is completed after the next complete spindlerevolution, the temporary RROC profile in the learning buffer 501 may bepromoted to the working buffer 502 as an updated RROC profile by, e.g.,swapping pointers. The updated RROC profile may be used for real timeservo control. Since the working buffer 502 is not updated until thenext complete spindle revolution is finished, it might not contain badRROC learning results.

During a high speed operation, track slips may happen during seeksettle. Track detection algorithms might not detect slips immediatelywithout any false detection, and there may be a detection delay toreduce the possibility of false detection. This detection delay maycause the RROC profile to be corrupted if a single-buffer adaptive RROCarchitecture is used.

With the dual buffer based iterative learning architecture of FIG. 5,once a track slip is detected during the RROC learning, the learningprocess may be stopped immediately. Since the track slip detection delaywindow is less than one revolution, the learning mistake due to thetrack slip detection delay may be saved in the learning buffer 501, butmight not be updated to the working buffer 502. This approach providesrobust protection for the RROC profile in the working buffer 502.

Thus, the architecture shown in FIG. 5 may update the RROC profile eachspindle revolution, while learning may take place per disc sector withineach spindle revolution. The learning process may be iterative perrevolution. If the learning process is less than 1 revolution before itis disrupted by any off-track condition, the learned RROC profile cannotbe moved to the working buffer 502 and cannot be used for servo control.

The control effort m may be contaminated with a DC signal 508 which maybe shared by all elements in the RROC profile. If the DC signal is notremoved, it may be saved in the memory buffers and affect accuracy ofthe servo control. In one embodiment, the RROC profile and theassociated DC signal may be learned at the same time and stored in thelearning buffer 501. Once a whole revolution is completed, the DC signalfor the whole RROC profile may be calculated at 506. When the RROCprofile in the learning buffer 501 is promoted to the working buffer502, the DC signal may be removed at 507.

FIG. 6 illustrates a flow chart of a method for reducing RRO accordingto one embodiment of the present invention.

At 601, a removable disc may be divided into a number of sectors (in oneembodiment, 128 sectors) with the method shown in FIGS. 1A and 1B.

At 602, an RRO profile may be obtained for each sector of the disc whenthe servo system shown in FIGS. 3 and 5 is in an open loop condition.

At 603, an RROC profile may be shaped for each sector of the disc.

At 604, an RROC profile for the whole revolution may be assembled fromthe sector RROC profiles, and saved in the working memory 502 forfeed-forward control.

At 605, the servo system shown in FIGS. 3 and 5 may be closed. When theread/write head is moving toward the target disc sector, z^−1, an entryin the previously stored RROC profile table corresponding to the targetdisc sector, may provide a feed-forward control effort n to suppressDrro for the target disc sector.

At 606, as soon as the read/write head reaches the target disc sector,the compensator C may provide a control effort m to reduce Dnrro for thetarget disc sector.

At 607, the low pass filter L may pass Drro in the error signal e toz^−1 for adaptation. Adaptation may continue until z^−1 for the targetdisc sector reaches a stable value when the control effort n cansuppress Drro, the error signal e does not have Drro contaminationanymore and/becomes 0. The stable z^−1 may be saved in the memory 501 asan update of z^−1 for the target disc sector.

At 608, it may be determined whether the head is moving to the next discsector. If yes, the process may return to 605.

Otherwise, at 609, it may be determined whether a whole revolution hasbeen completed.

If a whole revolution is completed, at 610, the temporary RROC in thelearning buffer 501 may be promoted to the working buffer 502, and maybe used in actual servo control.

FIG. 7 illustrates a removable disc storage servo system according toone embodiment of the present invention. As shown, a controller 701 maycontrol a read or read/write head 702 to access data on a removable disc703, e.g., an optical disc, by performing the method shown in FIG. 6.The controller 701 may access the working buffer 502 for the RROCprofile for actual servo control, and save a temporary RROC profile inthe learning buffer 501. The method of FIG. 6 may be implemented bycomputer instructions and/or hardware and the controller 701 may includehardware or firmware for executing the instructions.

While at one point a DVD system was used as an example in the foregoingdescription, the invention is not limited to an optical system. Otheroptical disc systems, including but not limited to CD-ROM and Blu-Ray™,and other removable disc systems may benefit from the inventive servotechnique.

Several features and aspects of the present invention have beenillustrated and described in detail with reference to particularembodiments by way of example only, and not by way of limitation.Alternative implementations and various modifications to the disclosedembodiments are within the scope and contemplation of the presentdisclosure. Therefore, it is intended that the invention be consideredas limited only by the scope of the appended claims.

What is claimed is:
 1. A method comprising: learning repeatable runoutprofiles of at least two sectors in a removable disc; generatingrepeatable runout control profiles for the at least two sectors based onthe repeatable runout profiles; assembling the repeatable runout controlprofiles into a single repeatable runout control profile; storing thesingle repeatable runout control profile for the removable disc in asecond memory; suppressing a repeatable runout of the removable disc viaa feed-forward control loop, wherein operations of the feed-forwardcontrol loop are performed in a time domain; storing a feed-forwardcontrol value in a second memory to adaptively adjust the singlerepeatable runout control profile, wherein the feed-forward controlvalue matches the repeatable runout of a target disc sector;transferring the single repeatable runout control profile from thesecond memory to a first memory subsequent to a spindle revolution; andremoving a direct current element from the single repeatable runoutcontrol profile prior to the transferring of the single repeatablerunout control profile.
 2. The method of claim 1, wherein: one of the atleast two sectors is a target disc sector; and the method furthercomprises suppressing a repeatable runout of the target disc sector viathe feed-forward control loop, wherein the suppressing of the repeatablerunout of the target disc sector is performed when a head reaches thetarget disc sector.
 3. The method of claim 2, further comprisingsuppressing a non-repeatable runout of the target disc sector via afeed-back control loop, wherein the suppressing of the non-repeatablerunout is performed subsequent to the head reaching the target discsector.
 4. The method of claim 3, further comprising separating therepeatable runout of the target disc sector from the non-repeatablerunout of the target disc sector.
 5. The method of claim 4, furthercomprising adapting a feed-forward control value to match the repeatablerunout of the target disc sector.
 6. The method of claim 5, wherein theadapting of the feed-forward control value is based on a repeatablerunout control profile of a sector adjacent to the target disc sector.7. The method of claim 1, further comprising stopping the learning whena head is off-track.
 8. A method comprising: learning repeatable runoutprofiles of at least two sectors in a removable disc; generatingrepeatable runout control profiles for the at least two sectors based onthe repeatable runout profiles; assembling the repeatable runout controlprofiles into a single repeatable runout control profile; storing thesingle repeatable runout control profile for the removable disc in asecond memory; suppressing a repeatable runout of the removable disc viaa feed-forward control loop, wherein operations of the feed-forwardcontrol loop are performed in a time domain; adapting a feed-forwardcontrol value to match a repeatable runout of one of the at least twosectors; storing the feed-forward control value in a second memory toadaptively adjust the single repeatable runout control profile;transferring the single repeatable runout control profile from thesecond memory to a first memory subsequent to a spindle revolution; andremoving a direct current element from the single repeatable runoutcontrol profile prior to the transferring of the single repeatablerunout control profile.
 9. A servo system comprising: a first memoryconfigured to store a repeatable runout control profile; a feed-forwardcontrol loop configured to suppress a repeatable runout of a target discsector of a removable disc when a head reaches the target disc sector,wherein operations of the feed-forward control loop are performed in atime domain; a compensator configured to (i) receive an error signal and(ii) provide a feed-back control value to reduce a non-repeatable runoutof the target disc sector; and a feed-back loop comprising a plantdevice configured to generate the error signal based on the repeatablerunout, and the compensator configured to (i) receive the error signaland (ii) generate the feed-back control value, wherein the feed-forwardcontrol loop comprises a low pass filter configured to receive thefeed-back control value, a first summer configured to sum (i) an outputof the low pass filter and (ii) an output of a finite impulse responsefilter, a feed-forward control device configured to generate afeed-forward control value based on an output of the first summer, andthe finite impulse response filter configured to receive an output ofthe feed-forward control device.
 10. The servo system of claim 9,wherein the low pass filter is configured to (i) separate the repeatablerunout from the error signal and (ii) pass the repeatable runout to thefirst memory.
 11. The servo system of claim 10, further comprising afeed-back control loop configured to connect an output of the firstmemory to an input of the first memory to adapt the repeatable runoutcontrol profile of the target disc sector.
 12. The servo system of claim11, wherein the feed-back control loop comprises the finite impulseresponse filter.
 13. The servo system of claim 9, further comprising asecond memory for temporarily storing the repeatable runout controlprofile prior to a whole revolution of the removable disc beingcompleted.
 14. The servo system of claim 9, wherein the compensatorcomprises an infinite impulse response filter.
 15. The servo system ofclaim 9, further comprising a phase lock loop configured to partitionthe removable disc into a number of sectors.
 16. The servo system ofclaim 9, further comprising a second summer configured to sum (i) thefeed-back control value and (ii) the feed-forward control value, whereinthe plant device generates the error value based on an output of thesecond summer.
 17. The servo system of claim 9, wherein the output ofthe finite impulse response filter is feedback from the finite impulseresponse filter to the first summer.
 18. A servo system comprising: afirst memory configured to store a repeatable runout control profile; afeed-forward control loop configured to suppress a repeatable runout ofa target disc sector of a removable disc when a head reaches the targetdisc sector, wherein operations of the feed-forward control loop areperformed in a time domain; a compensator configured to (i) receive anerror signal and (ii) provide a feed-back control value to reduce anon-repeatable runout of the target disc sector; a second memory fortemporarily storing the repeatable runout control profile prior to awhole revolution of the removable disc being completed; and a directcurrent signal remover configured to remove a direct current elementfrom the repeatable runout control profile stored in the second memoryprior to a transfer of the repeatable runout control profile from thesecond memory to the first memory.